In the oilfield industry, various downhole tools (e.g., packers, bridge plugs, frac plugs) may be used to isolate sections of a wellbore. These downhole tools generally include a central body or “mandrel.” Slips, a sealing element, and a set of components configured to expand the slips and sealing element are positioned on the mandrel so that the tool can be set, generally by application of an axially-directed, compressive force.
During setting, the slips expand outwards to grip the interior of a casing string (or another surrounding tubular in the wellbore), and the sealing element expands outwards to seal with the casing string. In the expanded state, the slips may maintain the position of the tool in the casing string, while the sealing element may isolate upper and lower portions of an annulus defined between the tool and the casing string.
The sealing element may be made from a deformable material, such as rubber. Such materials may, however, be prone to extrusion (e.g., axial expansion) along the mandrel during setting. Extrusion of the sealing element may reduce the ability of the sealing element to form a seal with the casing string. Thus, such tools are generally provided with one or more back-up rings, which are designed to prevent extrusion of the sealing element.
However, the back-up rings are generally constructed from soft materials, e.g., composites, to facilitate drilling through the tools when their use is complete. Back-up rings made from such soft materials may be prone to failure in the wellbore, such that the back-up rings may allow extrusion of the sealing element.